Fiber-reinforced plastics, which is one of the fiber-reinforced composite materials, have light weight, high strength, and high rigidity, and thus are widely used in products ranging from sports and leisure applications to industrial applications such as automobiles and aircrafts.
As a method for producing fiber-reinforced plastics, there is a method of using an intermediate material, that is, a prepreg, formed by impregnating a matrix resin composition in a substrate composed of long fiber (continuous fiber) such as reinforcing fiber. Such a method is advantageous in that the content of reinforcing fiber in fiber-reinforced plastics can be easily controlled and it is designed to have a large amount of reinforcing fiber.
Specific examples of a method for producing fiber-reinforced plastics from a prepreg include molding using an autoclave, compression molding, internal-pressurizing molding, oven molding, and sheet wrap molding.
Among fiber-reinforced plastics, fiber-reinforced plastic tubular bodies are widely used in sports and leisure applications such as fishing rods, golf club shafts, ski poles, or bicycle frames. With utilization of high elastic modulus of fiber-reinforced plastics, it is possible to hit a ball or a fishing hook a long distance with small force due to whip and reaction which occur at the time of swinging a tubular body. Furthermore, as light weight can be achieved by having a tubular body, operational feeling of a user can be improved.
In recent years, due to an increasing need for having light weight, it is attempted to use, as part of carbon fiber, fiber with higher elastic modulus, for example.
However, when carbon fiber with high elastic modulus is contained, fiber-reinforced plastics tend to have lower strength and are easy to break. As such, there is a limitation in use amount of carbon fiber with high elastic modulus. Furthermore, being highly expensive, the carbon fiber with high elastic modulus is disadvantageous from the economic point of view. Meanwhile, regarding fiber-reinforced plastics in which conventional carbon fiber is used, if the use amount of a prepreg is lowered to reduce the weight, the fracture strength of a tubular body is deteriorated.
Under the circumstances, the fracture strength of a fiber-reinforced plastic tubular body needs to be improved by a method other than the method based on modification of elastic modulus of carbon fiber.
To solve the problems described above, use of an epoxy resin composition as a matrix resin composition is suggested in Patent Document 1 and Patent Document 2, for example.